2.79 Multiple displace mods with different uv-maps and textures
After looking around I found my answer, but for anybody having the same problem:
it seems the maps "inherit" islands and textures from the previous map. I got it to work with choosing my maps, resetting (Unwrap - reset) the islands and starting over.
I guess the best way would be to create all the maps you need prior to unwrapping any faces to make sure you don't have any unwanted "leftovers".